1. Field of the Invention
The present invention relates to an actuator control device and an actuator control method which control actuators used for driving or the like of joint parts of a robot, an actuator, a robot apparatus, and a computer program, and particularly, to an actuator control device and an actuator control method which control driving of joint actuators by a force control method which directly controls a joint generation force, an actuator, a robot apparatus, and a computer program.
More specifically, the present invention relates to an actuator control device and an actuator control method which perform force control suitable for physical interaction with a human being while coping with factors of disturbances whose modeling or identifying is difficult, such as friction or inertia, which exists in joint parts, an actuator, a robot apparatus, and a computer program, and particularly, to an actuator control device and an actuator control method which perform force control suitable for physical interaction with a human being while coping with a disturbance problem by calculating a joint force for generating a desired force in a predetermined part of a body while the dynamics of the robot is taken into consideration, an actuator, a robot apparatus, and a computer program.
2. Description of the Related Art
With economic development, the mechanism of a decrease in the birthrate or stabilization of young population to a low level, and a decrease in mortality below the middle age acts. As a result, the phenomenon in which elderly population increases relatively proceeds rapidly. Although the ratio of elderly people and productive-age population which supports the elderly people was 1 person:3.3 persons at the present time of 2005, it is expected that the ratio will be 1 person:2.4 persons in 2015, and 1 person:2.1 persons in 2025. In such an aging society, it is imperative to make the elderly people live as healthily and actively as possible without requiring nursing care, or to make a society in which aggravation can be prevented as much as possible even if the elderly people require nursing care, and they can lead independent lives.
Additionally, the need for mechatronics instruments aiming at assistance in elderly people's minds and bodies is increasing in elderly-people nursing care facilities or at homes which have elderly people.
For example, an odor emission source detecting and eliminating apparatus is suggested which can be utilized for immediately sanitary removal of excrement which may cause odors in elderly people nursing-care facilities, hospitals, etc., and cause hospital infections (for example, refer to JP-A-2005-61836). Additionally, an action expression system is suggested which makes the action of an elderly person expressed by a robot at a close relative's house on the basis of a first detection result obtained by detecting the action of the elderly person, estimates the degree of interest of the close relative from a second detection result obtained by detecting the watching action of the close relative, and feeds back the result to give a change to the action expression of the robot (for example, refer to JP-A-2006-178644).
In recent years, the need for mental assistance in which robots are effectively incorporated in occupational therapy as well as physical assistance called power assistance of autonomous walking aids or upper limbs is further increasing.
The mechatronics instruments to be applied in the field as described above should execute a task while making physical contact with people or complicated actual environments flexibly and safely. That is, unlike the fact that related-art industrial robots performed fixed motions under a known environment, the inventors consider that the above mechatronics instruments should sense an unknown environment and obtain a proper external force from surrounding environments which vary every moment, thereby properly adjusting the generation forces of actuators so that a desired task may be achieved.
Here, the robot is basically a multi-link structure including joints which are a plurality of links and link movable portions, and is adapted so as to drive each joint by using an actuator including a DC brushless motor or the like. Additionally, the method of controlling such a robot (or joint actuator which constitutes the robot) includes, for example, positional control and force control. The positional control is a control method of driving the joint so as to give a position command value, such as an angle, to the actuator and follow the command value. The other force control is a control method of directly receiving a target value of the force to be applied to an operation object and realizing the force indicated by the target value, and can directly control a joint generation force and directly control a force.
Most of related-art robot apparatuses are driven by positional control from simplicity on control, and easy construction of a system. However, the positional control is commonly called “hard control” because its basic purpose is to hold a position, and is not suitable for responding to an external force flexibly or for precisely performing “soft” control in terms of velocity or acceleration. For example, robot apparatuses which execute a task while performing physical interaction with various outsides originally have low compatibility with the positional control.
In contrast, in the force control it is considered that more flexible physical interaction service with a human being in terms of force becomes possible although a control law or a system configuration becomes complicated. On the other hand, the force control is apt to be influenced by a disturbance, and the control is not easy.
Friction or inertia which exists in a joint part becomes a biggest problem as a disturbance in the force control. Parameters which determine the dynamics of the robot are classified roughly into mass properties, such as link weight, center of gravity, and inertia tensor, and the friction and inertia inside a joint. The former mass properties can be calculated relatively easily and with high precision on the basis of CAD (Computer Aided Design) data. In contrast, the latter (especially friction) is hard to model and identify and becomes a factor which causes a large error.
A method of coping with the disturbance problem in the force control can be classified roughly into following three methods.
As a first method, it is conceivable to perform the design of eliminating a deceleration mechanism which becomes as a generation source of unknown friction as much as possible from a joint part. Specifically, the first method includes using a direct drive motor (for example, refer to JP-A-2007-124856) or a configuration (for example, refer to U.S. Pat. No. 5,587,937) which performs deceleration of a low reduction gear ratio using a wire mechanism. According to this method, although high-response force control becomes possible, there is a problem that the motor becomes bigger or a sufficient joint force is not obtained.
As a second method, a method of arranging a force sensor at a working point of a force, feeding back the deviation between a generation force and a force command value to a joint command value, and suppressing a disturbance exerted on the generation force at the action point is conceivable (for example, refer to “Robust Motion Control by Disturbance Observer” (Journal of Robotics Society of Japan, Vol. 11, No. 4, pp. 486-493, 1993) by Onishi). According to this method, it becomes possible to suppress all disturbances, such as a disturbance force received from a cable as well as the disturbance inside the joint, whereas there is a constraint that interaction is limited to a part in which the force sensor is set. In other words, in order to allow interaction in an arbitrary part of a whole body, expensive force sensors should be mounted in various places of the body.
As a third method, a method of calculating a joint force for generating a desired force in a predetermined part of the body while the dynamics of the robot is taken into consideration is conceivable (for example, refer to “A prioritized multi-objective dynamic controller for robots in human environments” (In Proceeding of the IEEE/RSJ International Conference on Humanoid Robots, 2004)). For example, in the specification of Patent Application No. 2007-272099 already transferred to the present applicant, a control system is disclosed which determines generation force target values of all actuators by detecting contact parts with the outside without omission, using contact sensors arranged in a distributed manner over the whole body surface of a link structure, such as a robot, and by exactly solving a mechanical model so that a desired motion may be achieved, while properly using external forces having the detected contact parts as points of action. According to the control system, favorable tactile force interaction in which points of action are not limited can be realized by compensating a force, which is difficult to model, within each joint which connects between links by a torque sensor provided at a joint part.
However, in the third method, it is necessary to use an “actuator which makes an ideal behavior” for the joint part so that the friction and inertia of the joint part which become key factors of errors match a theoretical model. Additionally, it is assumed that a reducer is included in the actuator and that the actuator makes a behavior according to the theoretical model as a system including the reducer.
According to the third method, since errors of parameters other than parameters inside a joint can generally be made small, the precision of a generation force also becomes favorable. It becomes possible to utilize a reducer, and it also becomes possible to obtain a large joint force by a small motor. Additionally, since the force feedback from a force sensor is not used, it is also not necessary to mount force sensors in various places.